Four-year-old Libby and her month-old sister Connie were born thanks to an egg yolk.

Their parents, Mark and Suzanne Harper, first tried to start a family right after they got married, 12 years ago.

But heartache was followed by heartache, including two miscarriages and more than $50,000 on in vitro fertilization attempts that nearly bankrupted the family.

After Suzanne, now 36, didn’t get pregnant after a year, the couple, from Ilkeston, England, discovered Mark had zero sperm count because of an earlier battle with testicular cancer.

Worse news followed when Mark’ was stricken a year later with a different form of testicular cancer. “We were absolutely devastated,” Suzanne said.

Mark underwent chemotherapy and was declared in remission. Then in 2006 the Harpers tried artificial insemination with sperm from an American donor.

That, too, failed, so the couple tried IVF at a Nottingham facility called CARE. The first three tries failed and tests revealed why: Suzanne’s immune system was producing killer cells that attacked embryos.

Even so, on the couple sixth IVF attempt she did get pregnant — “and then I miscarried after six weeks.”

The CARE facility suggested a rare technique: treating Suzanne with “intralipid,” a mixture of the yolk from a hen’s egg and soy oil. In theory, the fatty acids were supposed to inhibit the killer cells that attacked her embryos and bring her to a healthy pregnancy.

The Harpers were surprised by the main ingredient. “But we were willing to try anything to try and have a baby,” Suzanne said.

On her eighth IVF try in April 2009 Suzanne became one of the first women in the United Kingdom to undergo the treatment.

It worked: Libby was born in December 2009.

But the Harpers wanted another child. Three more IVF attempts were made with the yolk emulsion. Two failed and the third resulted in another miscarriage.

“It was heartbreaking,” Suzanne said. “We did have our daughter but we were desperate to give her a brother or a sister.”

They tried one last time, in March 2013, and they were rewarded when Harper gave birth to 7-pound Connie in December.